Galactic population synthesis of radioactive nucleosynthesis ejecta

Vol. 672
6. Insterstellar and circumstellar matter

Galactic population synthesis of radioactive nucleosynthesis ejecta

The projected (line-of-sight) distributions of the short-lived radioactive nuclei ²⁶Al and ⁶⁰Fe have been detected in the Galactic interstellar medium with gamma-ray satellite observations from COMPTEL and INTEGRAL/SPI. This paper is a clever global simulation of those maps, using a range of formal representations for the spatial distribution of star formation (combinations of radial exponential profiles and spiral structure imposed on a disk) and with careful rendering of the underlying sources and their stochastic variability. While this is not ab initio modeling, the details of the in-out processes have been individually evaluated before being combined with a population synthesis at the Galactic level. A particularly attractive feature of this approach is that the lifetimes of the two species are so short that large-scale redistribution from macroscale dynamics within the interstellar medium is not a major concern. Therefore, unlike pre-solar grains, the simulations can exploit the "snapshot" approach to obtain a statistically consistent sky distribution. The authors find that a spiral-dominated disk (with a weak exponential radial dependence) with a scale height of about 0.7 kpc and a moderate star formation rate, about 8 M_sun yr, provides the best fit. This approach should encourage more detailed modeling and communication between the different specialties regarding Galactic chemical and population evolution.